FR2633049A1 - METHOD AND DEVICE FOR NON-DESTRUCTIVE CONTROL OF RAW BAGS DURING THE POWDER PRESSING PHASE - Google Patents

Abstract

The invention allows the non-destructive testing of green pellets, obtained by pressing powders, during the three pressing phases. A fixed die 2 is used formed of a central barrel 4, into which the powder to be compacted is introduced, and of a peripheral tire 6. An upper piston 8 and a lower piston 10, each have a translational movement. towards the other inside the central barrel 6 to compact the powder. The invention consists in placing at least one acoustic wave sensor 12 on the tool, which is made of a material of sufficient hardness to then perform the waveguide function, so as to continuously detect and follow the appearance and propagation of pellet defects. Application to the nuclear industry.

Description

NON-DESTRUCTIVE CONTROL METHOD AND DEVICE

  OF FLAKES IN THE PHASE

POWDER PRESSING

DESCRIPTION

  The invention relates in the field of powder metallurgy, the control of pellets obtained by compacting these powders. It relates more particularly to both the process and the non-destructive testing device during the pressing phase of the pelletized pellet powders.

the raw state.

  There are methods and devices for producing pellets which can be of very high density, such as those used for the nuclear industry. The pellets of this kind may for example be of uranium oxide, and have a high density approaching the theoretical maximum density. In this area, the non-destructive testing of raw pellets constitutes a

  problem that is solved only partially.

  Indeed, the methods commonly used are visual and geometric examinations of the pastille performed. Thus, external defects are detected by observing open cracks and open porosity. It is also possible to control dimensions after the pressing phase. These different controls are performed on the raw pellet. It therefore requires an additional duration of control, which is

  penalizing in the manufacturing cycle.

  - Moreover, these methods do not allow to detect internal defects. Currently, the detection of these is only by an ultrasonic method, applied to the finished product, that is to say after the powder pressing steps and pasting sintering. Again, additional time increases the time and cost of manufacturing. The object of the invention is to avoid the aforementioned drawbacks, and to control the green pellets, before-end of manufacture, and in particular during the pressing phase of the powders

of these pellets.

  It is recalled that a device for pressing such powders mainly comprises a matrix which itself consists of a central barrel inside which the powder is placed, this barrel being surrounded by a peripheral bandage. The compression is obtained using two pistons compressing the powder to

inside these cannons.

  The first main object of the invention is therefore a nondestructive testing method during the powder pressing phase in three stages of compression, decompression and demolding, giving green pellets. It is done by means of a tool composed of a fixed matrix, formed of a central barrel into which is introduced the

  powder to compact, and a peripheral bandage.

  It is also composed of an upper piston and a lower compression piston each having a translational movement towards each other at

  inside the central barrel to compact the powder.

  The invention is characterized in that the method consists of placing at least one acoustic wave sensor on the matrix, the central barrel and the peripheral bandage of the fixed matrix being made of a material of sufficient hardness to then perform the function. waveguide, so as to detect and continuously monitor the appearance -and the propagation of defects in the pellet during

of the three steps.

  Such a method. enables the continuous detection and tracking of the appearance and propagation of defects, both external and internal, of

  the pellet during the compaction phase.

  It is therefore possible to know with great precision the state of the pellet at the end of the compaction phase. If there are defects, their origin, namely Compression, decompression or demolding, and their nature, in particular cracks, lamination, or end caping, may

to be detected.

  A main embodiment of the invention provides that the acoustic wave sensor is

  consisting of a piezoelectric sensor.

  Several variants of embodiment provide

  different locations of the sensor or sensors.

  This or these sensors can be placed either on the bandage, on the central barrel or on the pistons. Several combinations of locations

  for these sensors are therefore envisaged. -

  A second main object of the invention is a control device for implementing the method that has just been described. It mainly comprises a sensor which delivers an electrical signal, and a processing chain of this signal

in view of its exploitation.

  The invention and its characteristics will be

  better understood by reading the description that

  follows, and which is a non-limiting example of a press.

  The appended figures represent respectively.

  - Figure 1, a first embodiment of the device according to the invention, without the measured signal processing chain; FIG. 2, a second embodiment of the device according to the invention, without the chain of processing the measured signal; FIG. 3, a diagram of said measured signal processing channel in the device

according to the invention.

  The method and the control device according to the invention will be described simultaneously in

the description.

  The manufacture of pellets, in particular of very high density, obtained by pressing of powders, requires the following tooling, represented in FIGS. 1 and 2. It comprises three essential elements, which are a fixed matrix 2 and two pistons 8 and 10 for compress the powder. The die 2 is formed of a central barrel 4, whose inside diameter corresponds to the diameter of the pellets to be obtained, and in which a quantity is placed.

  determined powder corresponding to a pellet.

  The matrix 2 also consists of a peripheral bandage 6, surrounding and holding the central barrel 4. The matrix 2 is held fixed on a table 14 by means of a fastening system which may be a first annular flange 16, which clamps the die 2 against a shoulder 17 of the table

{ix 14.

  The upper piston 8 is fixed, by means of a second annular flange 24, on an upper table 16. The latter is mounted movable in translation relative to the fixed table 14. This translation is a sliding of the upper table 16 on several guide columns 26, integral with the fixed table 14. The translational movement is obtained by a jack, whose foot 18 of the rod is fixed to the upper part of the upper table 16. Similarly, a lower table is movable relative to the fixed table 14, by means of the same guide columns 26. This lower table 20 supports a lower piston 10, fixed by means of a third annular flange 26. The same kind of translational movement is obtained thanks to a second cylinder whose foot 22 of the rod is attached to the lower part of the table

lower 20.

  The two pistons 8 and 10 are positioned so as to be able to penetrate inside the central barrel 4, when the upper table 16 descends

  and when the lower table 20 goes up.

  To obtain the compression of the powder placed inside the central barrel 4, the lowering movement of the upper piston 8 must correspond to the upward movement of the lower piston 10. These two movements continue until the distance separating them two pistons is equal to the thickness of the pellet to be obtained. The two pistons 8 and 10 each have a respective active part 9 and 11, whose diameters are equal to the diameters

interior of the central cannon 4.

  The control method according to the invention uses the acoustic measurement, during the manufacture of the pellets. For this purpose, at least one sensor 12 is fixed very rigidly on the matrix 2. This is a material of sufficient hardness

  to fulfill the function of sound waveguide.

  In this way, during the three compaction operations, the acoustic waves, due to these three operations, are transmitted through the whole of the fixed matrix 2, thanks to the very high hardness of the material which constitutes it. This may be in this case, refractory steel, tungsten carbide, or any other equivalent material from the point of view of hardness. The sensor 12 placed on the fixed matrix

  2 therefore receives these acoustic waves.

  The sensor 12 transmits its signal by means of a cable 30 to a processing line of this measured signal. Preferably, this sensor 12 is a piezoelectric sensor. The treatment chain

will be described later.

  With reference to FIG. 2, other possibilities of positioning the sensor (s) 12 are envisaged. It may be more efficient to place the sensor 12 directly in contact with the central barrel 4. For this purpose, holes 32 may be formed through the peripheral bandage 6, to allow the cables 30 to connect the wires.

  sensors 12 to the signal processing chain.

  It is also envisaged to place the sensor or sensors 12 very rigidly on one or both pistons 8 or 10, or on parts connected to these pistons in a very rigid manner. In this case, orifices 34 are provided inside the tables

upper 16 and lower 20.

  Several placement combinations of these sensors are therefore possible depending on the number

  sensors that we want to install.

  Supporting means 28 are provided to keep the sensors in permanent contact with the tooling. They may consist of a leaf spring 29, whose free end applies the sensor 12 to the tool. With reference to FIG. 3, the signal processing chain may comprise

the following elements.

  A gain pre-amplifier 40 may be used, to receive the measured signal, via the cable 30. It may be associated with a filter 42 whose bandwidth corresponds to that of the sensor 12, in this case a piezoelectric sensor. A variable gain amplifier 44 can then be used, in association with a filter 46, to eliminate high frequencies

  constituting the electronic noise of thermal origin.

  The exploitation of the results thus provided is done by means of a discriminator 48 completing this chain and an arcs or events counter 50. The chain ends with an acquisition system 52 which can be recorder-type and / or

  computer, in this case a - microcomputer.

  With the aid of an acquisition system 52 of the computer type, it is possible to work in real time, which makes it possible to know, during the pressing, the counting cycle giving the quality of the raw pellet made. It also makes it possible to work in offset time, where all the information collected is processed and analyzed, in order to identify the different mechanisms responsible for the defects, during each step of the powder compacting phase. The different measurable parameters, using this measured signal processing chain, are the number of events, the total number of pulses during an event, the duration of the event, the count rate per unit time, the amplitude of the events, and the

effective value of the amplitude.

  This control allows the immediate classification of the green pellets into compliant and non-compliant pellets; it also makes it possible to immediately make any modifications to the parameters of the pressing or the powders in order to obtain

the required quality.

  The method and the control device according to the invention make it possible to detect and continuously monitor the appearance and propagation of defects on the green pellets. These detected faults can be both external and internal, and occur during the next three stages of compaction, which are compression, decompression and demolding. The state of the raw pellet thus manufactured can therefore be known with great precision after pressing. The defects can thus be analyzed, and in particular their origin and their nature. One thinks in particular of the appearance of cracks of lamination, and of the phenomenon called in English language "end caping", corresponding to the

  formation of a crust on the surface of the pellet.

  It has also been noticed, during the tests, that the mode of elaboration of the powders obtained by wet way or dry way is identified thanks to the acoustic emission. It should be noted that the invention is not limited to the nuclear industry, but also applies to other fields outside the nuclear sector, for example, the manufacture of tablets in

the pharmaceutical field.

  It should be noted that this control method can be applied to all the raw pellets of a production unit or statistically to certain

pastilles of a Lot.

Claims (6)

  1. Non-destructive testing method during the powder pressing phase, in three stages of compression, decompression and demolding, by means of a tool composed of a fixed matrix (2) formed of a central barrel (4) ), in which is introduced the powder to be compacted, and a peripheral bandage (6), the tool also being composed of an upper piston (8) and a lower piston (10), each having a movement of translation to one another inside the central barrel (6) for compacting the powder, the method being characterized in that it consists in placing at least one acoustic wave sensor (12) on the tooling , said tooling being of a material of sufficient hardness to then fill the waveguide function, so as to detect and continuously monitor the appearance and propagation of defects in the pellet, during the three stages of compaction . 2. Method according to claim 1, characterized in that the sensor (12) is a piezoelectric sensor. 3. Method according to claim 1, characterized in that the sensor (12) is placed on the circumferential bandage (6) of the fixed matrix (2), the central barrel (4) and the peripheral bandage (6) being in one material of sufficient hardness   to perform the waveguide function.   4. Method according to claim 1, characterized in that the sensor (12) is placed on the central barrel (4), in orifices (32) of the circumferential bandage (6), the central barrel (4) - 11 being in a material of sufficient hardness to   then fill the waveguide function.   5. Method according to claim 1, characterized in that the sensor or sensors (12) are fixedly secured to at least one of the pistons (8, 10).   6. Control device for the implementation of the method according to any one of   preceding claims, characterized in that   comprises at least one sensor (12) delivering a measured signal, and a processing line of said measured signal